If you have the SEF urls of the form example.com/index.php/some-page.html you will almost always get this issue.
The best workaround is to set the live_site variable within the configuration.php file.
live_site = 'http://www.example.com', // without a slash at the end I believe.

this problem is with link give in joomla template design for the css.
open the index page of any joomla templates like beez, and same time open the index page of template which you have problem with it.
compare both templates in the header section for the css links you may find like this